1. Field of the Invention
The present invention relates to an optical pickup device and to a correcting element used in the optical pickup device.
2. Description of the Related Art
In recent years, there have been made research and development on the so-called high density optical disc wherein recording density of an optical information recording medium (optical disc) has been enhanced by using a blue laser beam having a wavelength of about 400 nm, and thereby, a storage capacity has been increased.
As a standard of the high density optical disc, there are known, for example, the one wherein an image-side numerical aperture of an objective lens (NA) is about 0.85 and a protective substrate thickness is about 0.1 mm and the one wherein NA and a protective substrate thickness are controlled to be respectively about 0.65 and about 0.6 mm which are similar to those of a conventional DVD (digital versatile disc). In the following explanation, the high density optical disc whose NA and protective substrate thickness are respectively about 0.65 and about 0.6 mm will be indicated as “HD-DVD (High Density DVD)”.
There are proposed various technologies relating to optical pickup devices each having compatibility between the high density optical disc of this kind and an optical disc which has been used widely such as DVD and CD (compact disc).
Incidentally, light flux wavelengths λ1, λ2 and λ3 used respectively for HD-DVD, DVD and CD are respectively about 400 nm, about 650 nm and about 780 nm, and protective substrate thickness t1, t2 and t3 are respectively about 0.6 mm, about 0.6 mm and about 1.2 mm.
To achieve compatibility among the optical discs in plural types of this kind, it is necessary to correct aberrations each being generated by a difference between wavelengths or a difference between protective substrate thicknesses, while securing an amount of a light flux used for each optical disc, and Japanese laid-open patent No. 2001-60336, for example, discloses a technology to provide a diffractive structure on an optical surface of an optical element that constitutes an optical pickup device.
The invention disclosed in Japanese laid-open patent No. 2001-60336 is an optical pickup device having an compatibility between HD-DVD and DVD or an compatibility among HD-DVD, DVD and CD, wherein chromatic aberration of HD-DVD is corrected by combining a diffractive optical element and an objective lens.
However, in the high density optical disc such as HD-DVD, a tolerance of a value of chromatic aberration in a light-converging spot is narrower than that of DVD, and it is necessary to secure an amount of light sufficiently, thus, in the invention disclosed in Japanese laid-open patent No. 2001-60336, correction of chromatic aberration has not always been sufficient.